Partners in Crime
by alyssss
Summary: Jace is sent on a mission to London, to track down Sebastian and stop him before he ruins the world. Here he encounters Sherlock Holmes, a detective with a dark and mysterious past, and John Watson, a lovable yet undeniably mundane sidekick. Can they defeat Sebastian and save the world of the Nephilim before it's too late? (JohnLock, spoilers for both series)
1. Chapter 1

"Sherlock?" John reached forward, resting his palm gently on his companion's elbow. "What's going on?"

The taller of the two reached into his coat and withdrew a long, thin knife from a pocket hidden under his arm. "Get behind me, John."

"Sherlock?"

A hooded figure had stepped from the shadows, a bloodied blade clutched in his pale fingers. As the man stepped forwards out of the alleyway and into the harsh fluorescent light of the street, he shook his head. The thick black hood fell backwards, and a mop of dirty blonde hair flopped forwards over his eyes. He turned the blade in his hand, cocking his head to one side as though to consider the pair of men stood before him.

"It's true then." His drawling accent had a slight American tinge to it, and he pursed his lips as he took another step forwards, stepping into the gutter. With every word, he edged forwards across the road, the blade turning over and over in his fingers. "You can't hide from the Clave forever, _Sherlock_. One would have thought you'd try and shield yourself from the public eye, but you've gone and done it this time. Well I've got some news for you." The man had reached the pair now, and this close John could see that he was little more than a boy. He was just as tall as Sherlock with cheekbones and a disposition to match, but his eyes were not quite as lined, and his jaw kept the slightly rounded shape of youth.

"What are you?" Sherlock hissed, the tip of his knife pressed against the neck of the youngster as he stood but an arm's length away from the detective. The harsh orange light of the streetlamps glinted off both blades, illuminating the smooth planes of the boy's profile as he laughed up into Sherlock's face.

"I'm one of _you_. And if you knew what's good for you, you'd take yourself and your little mundane friends off into the countryside. Lie low for a few years and let the _professionals _deal with this… _fiend_… you find yourself so occupied with."

"You say you are just like me- but you are naive. Inexperienced. I've fought my fair share of demons young boy, and by no means are you a professional when you compare yourself to _me_."

"Demons?" John let go of Sherlock's arm and stepped forwards, planting himself between the two men. "What's going on?"

The young boy pursed his lips once more and dropped the arm holding his knife to his side, rolling his eyes and pushing John back towards Sherlock with a flick of his wrist.

"I'd have thought you'd at least tell your _boyfriend_ what you are, Holmes."

Sherlock exhaled, dropping his knife to his side and pulling John behind him once more. "Now is not the time."

"Oh we all know why you left. Why you turned your back on the Nephilim. It's no secret, you know. You're not the first and you won't be the last, but you're in danger. You've got a target painted on your back, Sherlock. You and everyone you love are at risk. You're flying high now, but if you carry on in this way you'll _all _fall."

"Why should I believe _you_?" Sherlock's knuckles were white around the handle of his knife, his dark eyes stony and fixed on the face of the young boy before him. His voice trembled, his head shaking slowly from side to side as hot tears pricked at his eyes. This wasn't happening.

"Moriarty. That's who you're after, isn't it? He's working for Sebastian. Your mission tonight was a trap. He was going to kill you, take down you- and everyone you love- just like that. And you'd have walked right into it if I'd not turned up to stop you. Yet still, you refuse to listen to me."

"Why should I trust _you_? How do I know that you aren't working for him- this _Sebastian_?" Once more Sherlock stepped forwards, putting himself between John and the mysterious blonde American. His eyes burned with fury as he lashed out with his hand and gripped hold of the boy's neck with his slender fingers, lifting him gradually off the ground.

The boy choked, clawing at Sherlock's hands as his cheeks turned red. He kicked out with his foot and hit the detective in the chest, winding him and sending him stumbling backwards into the wall. Landing nimbly on his feet, he tucked his knife into his belt and bent over.

"Because if I were, I'd have killed you by now." Wrapping his hands around Sherlock's wrists, the boy pulled the detective upright and pushed him towards John, staring them down with a mischievous glint in his eyes. "Jace Herondale. Shadowhunter."


	2. Chapter 2

"Demons? Shadowhunters? What the bloody hell is going on, Sherlock?" John slammed the door behind Jace and shrugged his coat from his shoulders, throwing it swiftly over the closest armchair and stepping into the kitchen. "Tea?" He looked at Jace with a raised eyebrow, and Jace shook his head incredulously.

Sherlock threw himself onto the couch and lay with his feet dangling over the arm, one hand trailed on the floor while the other rubbed at the bridge of his nose. "Yes Sherlock, what the bloody hell is going on?" Jace added sarcastically, leaning against the wall next to the fireplace and running his finger through the thick coating of dust on the mantle.

"Enough of your cheek." Sherlock snapped upright, pointing his finger at Jace as he rolled his shirt sleeves up to his elbows. "Moriarty is an evil genius and he's been haunting me for years. He's a criminal mastermind and I'm determined to take him down, whatever it takes. The only problem is the press has gotten a little too attached to the case and the idea of my business in general. I'm overrun with mundane clients giving me stupidly mundane tasks and I'm 90% sure it's Moriarty that's put them up to it, so that I can't hunt him down and take him out once and for all."

John stepped from the kitchen, a tea tray in his hands. He balanced the wooden tray delicately on the paperwork-strewn coffee table in the centre of the room, and perched himself gently on the edge of his usual armchair next to the fire. "Demons." He put his hands on his knees, then changed his mind and ran his fingers through his thinning hair instead. His eyes darkened with confusion as he looked up, glancing from Sherlock to Jace and puffing his cheeks out in exasperation. "Demons. They're real? Really real?"

"Does he ever shut up?" Jace looked at Sherlock with wide eyes, his hands clutching the human skull that Sherlock insisted on proudly displaying in his home.

Smiling slightly, Sherlock shook his head and got to his feet, shaking out his limbs as he turned to face his friend.

"Demons. Shadowhunters… Where to begin?" He laughed nervously, fiddling with his sleeves once more before shoving his hands into his pockets and beginning to pace the room. "You- yes. It's easier if I begin with you. You, John Watson, are a mundane. You're human. As human as humans can be." Sherlock was talking with his hands, waving them frantically back and to as he paced along the rug. "I'm also human, though not completely. I share the blood of the angel Raziel. I'm a Shadowhunter, a Nephilim. Like him." He pointed to Jace, who waved meekly from where he stood and brushed his dusty hands off on his trousers, combing his hair out his eyes with his fingers. "We're fighters. Born to protect the human race from the dark forces. Demons. They're alien. Not of this world… and they're evil, generally. I've never met a good one in all my time on this earth and I'll bet none of our people ever have, but they're everywhere."

"And Moriarty? He's one of them?" John asked, his head still in his hands. Sherlock knelt on the ground before him and reached forward with his hand, cupping John's chin gently in his fingers and raising his face so that he could look into his eyes.

Sherlock's face was glowing with excitement, his eyes glistening with pride and understanding. "No, John. He's one of us."

"_Actually_," Jace said, stepping forwards. "Not strictly true. He's a mundane- a human. He's just doing Sebastian's bidding."

Sherlock leaned closer to John, pressing his forehead against that of his companion and running his hands around to grip hold of his hair. John could feel Sherlock's breath on his cheeks, hot and fast as his mouth moved on. "We can take him down. We can fight him." He laughed, letting go of John's hair and standing up again, spinning on the spot and slipping a thin silver pencil-like object from his pocket. He put out his wrist and traced a shape onto his skin with the stick, still laughing as black lines began to snake outwards and over his skin, sliding under his shirt and visible through the thin white fabric as they settled themselves onto his chest.

John looked at the shapes with eyes of question and wonder, for where thin white scars used to litter Sherlock's skin there were now intricately twisting tattoos of the likes he had never seen. He coughed, rubbing his hands nervously on his trousers. The tea was forgotten as Sherlock slipped the stick back into his pocket and looked at Jace with bright, shining eyes. "We have to go. Now. If we carry on as we were- if we take a cab across London to St Pauls and then.."

Jace shook his head, stepping across the room and, calmly removing Sherlock's coat from his hands, hung it back up on the hooks next to the door. "There are a number of reasons why that's a bad idea. Firstly, it's three in the morning. Secondly, I'm horribly jet lagged and would be absolutely rotten in a fight. Thirdly, you're insane."

Sherlock rolled down his sleeves and looked grudgingly down at John, who blinked up at him blankly. "He has a point." He muttered, pouring himself a cup of rapidly cooling tea and finally leaning back into his seat. Sherlock shook his head and slouched off to the bathroom, while John awkwardly held his tea and looked up at Jace questioning. "Have you got a place to stay?"

Jace shrugged. "I've got money, I can get a hotel." John put his head to one side and smiled.

"It's 3 in the morning. We've got a spare room since uh…" Jace smiled at John's hesitation. "Since we started…" Laughing, Jace nodded and took the teacup from John's shaking hand and put it down on the cluttered coffee table.

"Relax. My brother's like you. Gay, I mean. I'm totally used to it. He's dating this warlock… well at least, he was…" Jace scratched his head theatrically. "I've no idea what's going on with those two. What was that about a room?"


End file.
